Torque limiting devices of various sorts are well known in many fields where mechanical power transmission is used. Such devices are generally found with various combinations of motors or engines and their associated transmissions, speed reduction systems, etc. The use of such torque limiting devices is especially valuable in systems in which the output shaft is subject to sudden stoppage or overload. Such a torque limiting device will slip in such a situation and protect the powerplant and/or transmission from potential damage due to the sudden stoppage or overload, as may be encountered in the operation of a boat should the propeller become fouled.
While this device performs adequately in terms of preventing damage to the remainder of the drive system of the boat due to torsion forces, in many cases an additional tension force is produced axially along the drive line of the boat due to the propeller being caught or snagged on a stationary object and the mass and inertia of the boat remaining in motion. On occasion an axial tension force may also be produced when a propeller picks up a rope or line, which is then wrapped around the rotating propeller shaft. This buildup of material around the shaft in combination with the tension on the line as it is drawn onto the shaft may cause the line to pack around the shaft, spreading longitudinally along the shaft between the propeller and bearing support or other structure and forcing the propeller out from that support or structure. The net result of such an incident is very often the same as that of the propeller catching on a stationary object, i.e., an axial tension force is applied to the drive system.
Typically, the drive system in such a boat comprises an engine, a reduction drive and/or transmission, a drive shaft, an external shaft bearing and support assembly, and a propeller. These components are typically assembled with one or more connecting flanges along the shaft between components. Such a system is generally not designed to accept large forces along the axis of the drive system; thrust forces which would produce a compressive axial force along the drive line are transmitted to the hull by means of the bearing support assembly. Generally, the axial tension forces generated by normal thrust reversal are insufficient to cause damage to any component due to the relative inefficiency of the propeller operating in the reverse direction as well as the generally smaller power output and slower speeds desired during such reverse operations.
Thus, when some fouling of the propeller occurs as described above the typical drive line is incapable of accepting the resulting axial tension forces and damage to one or more components occurs. The typical "weak link" in the system is the speed reduction gear box, which provides little in the way of axial load protection either in tension or compression. When such a reduction gear box fails due to tension loads along the drive shaft, the resulting damage is relatively costly and time consuming to repair, possibly costing an operator tens of thousands of dollars when the time the boat is out of commission is considered.
What is needed is a torque limiting clutch which is also capable of limiting the amount of axial tension force which may be applied along the drive shaft, thus protecting all other components in the system. Such a device should be relatively simple and inexpensive to repair, allowing a boat using such a device to be quickly repaired using commonly available parts with a minimum of lost time.